As phosphors emitting a visible light upon excitation with vacuum ultraviolet rays or ultraviolet rays, there are known silicate phosphors. For instance, there is known a blue light-emitting silicate phosphor having formula of Sr3MgSi2O8:Eu, namely, a blue light-emitting SMS phosphor. There is also known a green light-emitting silicate phosphor having formula of (Ba,Sr)2SiO4:Eu. Further, there is known a red light-emitting silicate phosphor having formula of Ba3MgSi2O8:Eu,Mn.
As light-emitting devices which emit a visible light from a phosphor excited with vacuum ultraviolet rays or ultraviolet rays, there are known an AC type plasma display panel (AC type PDP), a cold cathode fluorescent lamp (CCFL), and a white light-emitting diode (white light-emitting LED).
In the AC type PDP, vacuum ultraviolet rays produced by discharge of Xe gas are applied to all of the blue light-emitting phosphor, green light-emitting phosphor and red light-emitting phosphor, for producing a blue light, a green light and a red light. These lights are used to produce images. The emission produced by the discharge of Xe gas mainly comprises the Xe resonance emission and Xe2 molecular beam emission. The resonance emission comprises vacuum ultraviolet rays having a center wavelength at 146 nm (or 147 nm, according to some texts). The molecular beam emission comprises ultraviolet rays having a center wavelength at 172 nm (or 173 nm, according to some texts).
In the CCFL, ultraviolet rays produced by discharge of Hg gas are applied to all of the blue light-emitting phosphor, green light-emitting phosphor and red light-emitting phosphor, for producing a blue light, a green light and a red light. These lights are mixed to give a white light. The ultraviolet rays produced by the discharge of Hg gas have wavelength at 254 nm.
Most of known white light-emitting LEDs comprise a light-emitting semiconductor element producing a blue light upon application of electric energy as well as a phosphor-containing resin composition comprising a yellow light-emitting phosphor dispersed in a resinous binder. In the white light-emitting LEDs, the blue light emitting from the semiconductor element and a yellow light emitting from the yellow light-emitting phosphor upon application of the blue light produced by the semiconductor element are mixed to give a white light. This white light-emitting system is called “dual color-mixing system”. There is such problem, however, that the white light given by the dual color-mixing system is low in its color purity.
For the reason described above, a white light-emitting LED utilizing a triple color-mixing system has been developed. The white light-emitting LED according to this system employs a combination of a semiconductor element emitting an emission having a wavelength region of 350 to 430 nm upon application of electric energy and a phosphor-containing resin composition which comprises three kinds of phosphors, namely, a blue light-emitting phosphor, a green light-emitting phosphor and a red light-emitting phosphor, dispersed in a resinous binder such as epoxy resin or silicone resin. In this white light-emitting LED, the emission produced by the semiconductor element is applied to these phosphors and the blue light, green light and red light emitting from these phosphors are mixed to give a white light.
In the white light-emitting LED, silicate phosphors are employed. It is known, however, that the silicate phosphors are reactive with moisture, and hence are liable to decrease their emission strength upon contact with moisture. Therefore, it is desired to improve the moisture resistance of the silicate phosphors employed in the white light-emitting LED so as to keep them from decreasing their emission strength, which is caused by moisture permeated through the resinous binder.
There are a number of reports concerning the improvement of the moisture resistance of silicate phosphors in which the improvement is based on formation of a coating layer over the surface of the silicate phosphor.
D1 (JP 2011-68792 A) describes that the moisture resistance of a silicate phosphor can be improved by forming a moisture proof coating layer over the surface of the phosphor. The moisture proof coating layer comprises metal oxide particles dispersed in a metal oxide matrix phase, in which the metal oxide particles are plate-like particles having a particle size of 2 nm to 1 μm and a thickness of 100 nm or less under such condition that the thickness is less than ⅕ of the particle size.
D2 (JP 2002-539925 A) describes that inorganic material particles reactive with water can be made to have improved moisture resistance by forming a moisture impermeable-coating layer over the surfaces of the inorganic material particles. The moisture impermeable-coating layer can be formed by heating a mixture of the inorganic material particles or higher. The phosphor specifically mentioned in D2 to be improved in its moisture resistance is an alkaline earth metal aluminate. Further, the specification of D2 describes that ammonium fluoride to be calcined together with the phosphor is employed in an amount by weight of approx. 1:3 to 1:6, per the phosphor. The amount of ammonium fluoride employed corresponds to 16.7 to 33.3 weight parts, per 100 weight parts of the phosphor.